Again: Model T questions

[Das_TIER]

Hi guys,
I'm playing around with the idea of building myself a Model T, because I've got some experience in electronics and like sunn o)))
The clean tone is nice too...

Anyway, I figured I need to do it on the cheap (sort of) because i haven't got much money. I can't afford original transformers, even if I would find some in europe. And thats at the moment my main problem: can i just use some other power transformer + simple psu?
I mean a model T really only needs 330V, 500V and -51V, and there are many old power transformers for tube amps on ebay. I'll just get one that matches the given voltages as close as possible, and it wouldn't hurt if the power amp B+ was 400V instead of 500V, right?

Please correct me if i'm wrong.

And what about the OT? There is basically no information about it in the schematic. Is it the same as in the 98 reissue?
I need some specs to get a similar one...

I've got some questions on the '73 schematic, too.
1. The NFB loop only connects to R22 and R26/C10/C9, right? Its really hard to see if it connects to C8 or not. It wouldn't be logical, but its always better to ask than to screw up instead...
2. C21 says 2.2µF, 50V. I guess it should be 22µF as stated in another thread, but it clearly says 2.2µF in my schematic.


Please let me hear what you think, can this work?
Of course the sound will be different when i use different transformers, but I hope that its not going to cause issues and will basically sound like a model t.

cheers

[AjaxLepinski]

Mercury Magnets makes great transformers and they make Sunn clones!!! You are in like Flynn dude!!!
Here's the link:   http://www.mercurymagnetics.com/pages/mainframe.htm

Good luck with the build and post some pics!!!

[Das_TIER]

Thank you so much, man!
You made my day

800$ in total, I guess I have to start saving...
I'll certainly post some pics!

bye for now!

[_peter]

Hi Das_TIER,

are you sure you want to build such a high voltage tube amp with your state of knowledge?
You should be able to answer most of your questions yourself.

You don't need to buy the fancy overpriced mm transformers from oversea (you're also from
EU, right?) There's a SUNN power transformer available at TAD. You could also use their
Marshall Major OT (cheaper at banzai musik) or the Hammond 1650TA as output transformer.

I'm not sure about the cathode cap on V2. I attached a picture I found. Looks like 2.2µF to me.

R20 is NOT connected to C8. This is a typical long tail pair PI as found in literally hundreds of
other guitar amp circuits. Do you have someone experienced to help you building this amp?

Chears,
Peter

[Das_TIER]

Hi,
Well, first of all thanks for the info!
That'll certainly be an option, but to be honest, when I'm building a Project like this I want it to be as best as possible, and buying in europe would save me like 400€. Which is a lot, no one can regret that, but in perspective to the rest it doesn't make much sense. Did I mention that these mercury transformers are handwound by spaghetti monsters in moonlight?
But thanks anyway

Thanks for the pic! That clearly says "2.2MFD 50V".
Got the thing with R20 and C8 figured out by myself, thanks

I've got a few points of reference to build this amp.
1. My mother works in a Bookshop so she can get me whatever book I need on the topic with a discount.
2. I know a guy who is basically a tube-fanatic, he does all kinds of radio/tv repair since the 80s.
3. I've got physics teachers who can help me out with the math.


Anyway, in the meantime a new question has come up:
Should I use Aluminium or steel for the chassis?
I've heard that steel isn't the best choice because it transfers the magnetic energy from the transformers -> 50Hz hum in the OT. Is that an issue?


Btw. I've setup a Crowdfunding campaign -> https://www.gofundme.com/model-t-project
Share dat shit Thanks!


cheers

[_peter]

Hi,

as you live in Germany, for anything regarding guitar tube amps I striclty recommend you this forum:
http://www.tube-town.de/ttforum

The original Model T chassis is built of steel (568mm x 241mm x 89mm). Aluminium on the other hand,
is easier to work on. Don't know how big the difference in hum blocking is.

Good luck with the funding page.

Cheers,
Pete

[Das_TIER]

Ok, I'll use steel then because i already have a 19" rack case. (2U)
But I was rather suprised at the width! 56cm?!? My rack case is only 40cm wide... we'll see, if it doesn't fit I'll get a custom chassis made.

I've got another question on the schematic, this time regarding the PSU:
What types of diodes were used for B+?
I googled the given number "22-0416" and found some 2000V, 30A diode. I've heard a lot, but I can't believe that four 6550 draw anything near 30A... 

EDIT:
As the transformers aren't available anymore on TAD I have settled for Hammond 1650T and 378CX. Reference here (Post #14): http://www.diyaudio.com/forums/instruments-amps/268682-build-sunn-model-t-clone-some-questions-eu-2.html


Cheers


PS: _peter is auch dein Name im TT forum, stimmts?

[_peter]

Hi,

I don't know which diodes these are but two UF5408 will do just fine.

Regarding the Hammond: You know that the Hammonds secondary is tricky to wire up for
more than two impedances? For usual switching of 4-8-16 ohms via a rotary switch use
the 1650TA.

PS: _peter is auch dein Name im TT forum, stimmts?

Correct! 

[Das_TIER]

Yeah, saw that with the 1650TA, but I can't get one of those, Tube-town has only got the 1650T, and the other shops don't haven't even got hammond transformers (TAD, musikding, reichelt, conrad...). 

However, two new things have come up:

The power transformer seems a bit oversized upon closer examination.
800V RMS with C.T. is a bit too much, is't it? that would be like 1kVDC after rectification and a filtercap.
I therefore chose the 372LX, 600VAC RMS should be enough.
And then I discovered that the 372LX has got a 5V winding for tube rectifiers...

That's it, I'm sold.

Looking at a Sunn 2000S schematic (http://www.4tubes.com/SCHEMATICS/BY-BRAND/Sunn/2000s.gif), I could just run two GZ34's in parallel and get my 550VDC this way. (Do I even need that much? The 1. gen schematic isn't that clear, after two caps and a Choke it should be ~500VDC)
But then I need a smaller transformer, as the GZ34 is only rated for max. 550VAC (Plate to Plate).
The 370LX comes to mind...

So I'm basically stuck with the decision of my PT. What do I need, what considerations are there to make with tube rectification? The GZ34 Datasheet recommends ~200Ohm limiting resistors, but the sunn 2000S hasn't got them -> Are they really neccesary?.

AND EVEN if I go with diodes, the question remains: How much RMS do I need on the sec. HV winding?

This Project is getting more and more complex

cheers


PS: I've just finished building the preamp, I made myself a 300VDC supply with two 230V/12V transformers and use the master volume output (In the 1. gen schematic) to feed the FX-return of a Transistor-Amp (H&K Tour Reverb). SOUNDS F'IN AWESOME!!
Maybe I'll post some photos soon...

[_peter]

Dude,

800V RMS with C.T. is a bit too much, is't it? that would be like 1kVDC after rectification and a filtercap.

like I said above - you're lacking the knowledge to build such an amp. How can you dare to
build a high voltage tube amp when you don't get the difference between full wave rectification
with a center tapped transformer and bridge rectification?

I'm sorry and I know it's hard to hear but please stop this project, learn about fundamentals of
electronics and safety aspects and start with a smaller amp. This is how every amp builder became
an amp builder and you can't just take a shortcut. It would be irresponsible to encourage you to go
on with this any further.

[Das_TIER]

I can understand your concerns, but I'll don't take a shortcut. As I said, I've just built a 300VDC preamp, basically a Bassman and I learned a lot during that saturday night
I'll post some pics today, so you can see for yourself 

And I'm planning to build a Champ-like amp soon, practice what you preach

And I came here yesterday evening to tell you I just figured it out by myself
Its just that I don't have that much experience with tube amps. But I'm lerning, as you see.
Full wave C.T. is new to me, I only knew full wave bridge before, as its commonly used in low voltage projects.

And I've discovered a really informative video series on designing an amp -> https://www.youtube.com/watch?v=glThOrdGvmI
The amp designed is not that similar to a model T, but the methods used to analyze/design are the same.
Going to read a lot of books, too...

As the 378cx hasn't got a 5V winding, I'm going with UF5408 diodes.

cheers

"Feel the fear and do it anyway, Live!" - Doug Jackson (SV Seeker project)

[Das_TIER]

I've got some photos of the preamp...
http://uploads.im/OEiHW.jpg
http://uploads.im/Gf4zO.jpg
http://uploads.im/4kp1Z.jpg
http://uploads.im/13z6C.jpg
http://uploads.im/RLHBJ.jpg
http://uploads.im/AjhCX.jpg
http://uploads.im/WuiEU.jpg

Yeah, I know. I could have done a better job on the shielding, that input jack is way too close to the recto, but it works and is good enough for a saturday night.
Maybe I'll post some sound samples, but the thing just sounds like it should. Its pretty close to a 59 fender Bassman, look here: https://youtu.be/vgqrOCUQzH0?t=1m20s

cheers